During wood pulp production in a sulphate or Kraft process, a wood pulp-containing fibrous material is digested in a solution containing sodium hydroxide and sodium sulphide, which is known as white liquor. This digestion produces a wood pulp and also what is known as black liquor as a by-product. In a process for recovering the by-products in the Kraft process, unreacted chemicals or reaction products and in particular the recyclable components of the black liquor are recovered and used again for wood pulp production. To this end, the black liquor is firstly concentrated by evaporation, then incinerated in order to produce what is known as smelt. The smelt is dissolved and a green liquor is obtained; said green liquor contains sodium carbonate and sodium sulphide as the major components. The green liquor is then supplemented with quicklime or calcium oxide so that the sodium carbonate can be reacted to form sodium hydroxide and calcium carbonate in what is known as the recausticizing reaction in accordance with the overall equation Na2CO3+CaO+H2O═NaOH+CaCO3. The converted mixture is filtered through a filter on which solids, primarily calcium carbonate, also known as lime mud, are separated. The separated solids are diluted with water or filtrate from a downstream filtering stage, filtered again through a filter, transferred to the lime kiln and transformed into quicklime. The filtrate obtained is returned to the process as white liquor.
The reaction of green liquor with calcium oxide is an equilibrium reaction which in practice only goes to a conversion of 80% to 85% of the sodium carbonate, i.e. to a level of recausticizing of 80% to 85%. From the point of view of process economics, this low conversion level is dependent on various factors such as the total alkali concentration, the sodium sulphide concentration, the temperature as well as the excess of calcium oxide, for example. In particular, the amount of excess of calcium oxide which is necessary for as complete a reaction of the sodium carbonate to calcium carbonate and sodium hydroxide as possible has a disruptive effect on the subsequent process and in particular on the filterability of the lime mud, as well as on the downstream lime circuit in which calcium carbonate incinerated to regain the calcium oxide. The excess quantity of calcium oxide, which is usually a maximum of approximately 5%, sediments out, in particular onto the filter cloths and sieves, or gives rise to what is known as ring and ball formation in the lime kiln; these are associated with substantial losses and disruptions to the process.
A process for controlling the sodium carbonate concentration in the green liquor is described in EP 0 524 743 B1, for example, in which process, based on a measurement of the conductivity of a green liquor and a measurement of the conductivity and rate of flow of a washing solution, the sodium carbonate concentration of the green liquor can be adjusted or readjusted.
Furthermore, WO 85/01966 describes a process for recausticizing green liquor in which lime is added in a two-stage recausticizing process in a manner such that the lime which is used in a first recausticizing stage is of lower quality than the lime which is used in the second recausticizing stage; in this manner, it is hoped that as much sodium carbonate as possible is transformed into sodium hydroxide; the liquor can then be used in the paper manufacturing process.